1. Field of the Invention
The invention pertains to penetration-resistant material comprising a double layer of fabric with high linear density ratios of two sets of threads, and to articles made of the same.
2. Description of Related Art
Penetration-resistant articles such as bulletproof vests, helmets, vehicle panels, and shields prepared from high strength fibers are known in the art. For many applications, in particular for ballistic vests, the fibers are used in a woven or knitted fabric. These fabrics may be coated or impregnated in a matrix to obtain hard ballistic materials, or may be used free from matrix to obtain soft ballistic materials.
Bulletproof woven fabrics are known, inter alia, from EP 310,199. The fabrics disclosed therein are composed of filament yarns of ultrahigh molecular weight polymer having high strength and high modulus, with the warp threads being of a different polymeric material than the weft threads.
In Russian patent RU 2,096,542 a ballistic fabric for bulletproof jackets was disclosed having warp and weft threads of poly para-phenyleneterephthalamide (PPTA) wherein the ratio of warp to weft linear density is smaller than 4.17:1. Typically, warp threads having a linear density of 143 to 588 dtex and weft threads having a linear density of 588 to 930 were disclosed, the weft threads having equal or higher linear density than the warp threads. It is particularly contended that ballistic fabrics having warp to weft linear density ratios between 1.59 and 4.17 have improved deflection properties.
In WO 00/42246 a penetration-resistant material is disclosed comprising at least a double layer of fabric composed of two layers of woven fabric which are cross-plied at an angle wherein the fabric is composed of a first set of threads comprising 3.5 to 20 threads/cm and having a linear density of at least 420 dtex, and a second set of threads comprising 0.5 to 8 threads/cm and having a linear density of at least 50 dtex, with the second set of threads being transverse to the first set of threads, and wherein the ratio of the linear density of the first set of threads to the linear density of the second set of threads is >4.2:1, more preferably >7.5:1. In a preferred embodiment the first set of threads is warp threads made of p-aramid yarn and the second set of threads is weft threads of polyester yarn, and the ratio of the number of threads/cm of the first set to that of the second set is >1:1. Although the ballistic performance of this article is excellent, the necessity of cross-plying the layers is a disadvantage in terms of ease and simplicity of the manufacture and the danger of creating weak points, that inherently to the process of cross-plying can occur.